Patent GB-1481382-A (GB)
Title: Digital tacan system
IPC: G01S13/74
Filed: 1975-04-03
Abstract: 1481382 Tacan SINGER CO 3 April 1975 [5 April 1974] 13613/75 Heading H4D Digital apparatus to receive and decode a Tacan bearing signal comprises, Fig. 1, an antenna 11, an RF and detector stage 13 coupled to said antenna and feeding a matched filter 23; means 25 coupled to the output of said filter to detect the time of arrival of each signal pulse and to provide an output indicative thereof; means 27 to detect the amplitude of each signal pulse and to provide an output indicative thereof; means 33 coupled to the output of said filter to detect the time of arrival of a reference pulse and to provide an output indicative thereof; a first Kalman Filter 35 fed by means 33 to act as a reference phase tracker to provide outputs which are estimates of the phase and frequency of the bearing signal; a second Kalman filter 37 fed by means 25 and 27 and by the first Kalman Filter 35, to act as a bearing signal tracker to provide outputs which represent the five state variables of the received Tacan signal i.e. 15 Hz phase, 135 Hz phase, 15 Hz amplitude, 135 Hz amplitude, D.C. mean amplitude; and means 39 to compute from said state variables, output of said second Kalman Filter and the phase estimate output from said first Kalman Filter, the bearing to the transmitting station. The Tacan signal processor provides optimum signal processing employing the technique of a least square recursive (Kalman) filter to update the current bearing estimates in an optimum way with each new data input. To perform the bearing computation, the first Kalman Filter 35 provides a continuous estimate of the reference frequency # (15 and 135 Hz) and the reference phase #. The second Kalman Filter 37, acting as a bearing signal tracker, updates the five state variables. The use of digital processing techniques allows a shorter acquisition time, improved bearing accuracy at low signal to noise levels and automatic signal to noise monitoring. The implementation of the ranging function of the TACAN receiver involves an acquisition phase during which a coarse range estimate to a specific tolerance is made using a minimum number of interrogations. To allow this, the maximum allowable range designated R max  (normally R max  equals approximately 300 miles) is partitioned and completely covered by equal intervals or "bins" which are pre-selected as W miles wide. During acquisition, a record is maintained of the total number F of range pulses which have occurred in each bin and this record is updated after every new interrogation. The contents of the range bin corresponding to the aircraft range will increase by one count after each inquiry but all others will grow more slowly as determined by the random occurrence of the extraneous pulses received. To avoid problems of falling on the range between two bins, the number of bins can be doubled to provide overlapping bins. Once the course range has been determined within the tolerance of plus or minus W/2, fine tracking commences to improve accuracy and achieve and maintain a required accuracy of better than 0À1 mile, using a third simple Kalman Filter 41 of two state variables, range and range rate. A complex mathematical analysis is given.